1. Field of the Invention
This invention relates to a method for controlling the position and attitude of a satellite to prolong the longevity of satellite operation.
2. Description of the Related Art
Spacecraft propulsion systems normally use an array of bipropellant thrusters that burn a fuel in combination with an oxidizer to provide orbit raising, station keeping and attitude control. A typical thruster rocket for station keeping and/or attitude control is shown in FIG. 1, which includes a fuel tank 2, an oxidizer tank 4 and at least one thruster 6. Numerous thrusters can be included in a single rocket system, with thruster nozzles 8 pointing in different directions but sharing the same fuel and oxidizer tanks. In normal operation, the fuel tank contains a liquid fuel 10 and a helium gas combined with fuel vapor 12, and the oxidizer tank contains a liquid oxidizer 14 and a helium gas combined with oxidizer vapor 16. The helium gas in both tanks provides pressure to urge the liquid fuel and oxidizer out of the tanks. The fuel and oxidizer tanks are connected to the thrusters by respective fuel and oxidizer lines 18 and 20 and controlled by respective fuel and oxidizer isolation valves 22 and 24.
The normal operation of a rocket thruster is illustrated in FIG. 2. The fuel 10 and the oxidizer 14 are injected into a thrust chamber 26 via respective fuel and oxidizer lines 18 and 20. Oxidizer and fuel streams 28 and 30 injected into the thrust chamber are mixed together for combustion at a sufficiently high temperature. The heated gas 32 resulting from the combustion of the fuel with the oxidizer expands through the thruster nozzle 8 to produce a thrust.
The fuel and oxidizer lines, the thrust chamber and the nozzle generally have metallic walls. A heater plate 34 is attached to the metal forming the thrust chamber wall 36 to warm the thruster during non-operation, since the vaporization of fuel and/or oxidizer in the thrust chamber reduces the chamber temperature. Heating is not required during normal rocket propulsion when the fuel and the oxidizer combust in the thrust chamber.
The oxidizer is typically a volatile, relatively high density fluid that comprises approximately 97% Nitrogen tetroxide (N.sub.2 O.sub.4) and 3% mixed oxides of nitrogen with a freezing point of about 12.degree. F. The fuel is typically monoethylhydrazine (MMH), which is less dense than N.sub.2 O.sub.4 and has a lower vapor pressure point. A spacecraft rocket thruster requires both propellants, i.e., the fuel and the oxidizer to burn the fuel in the thrust chamber and generate a propulsive force. Mission planning and/or the occurrence of anomalies can lead to situations in which the fuel and the oxidizer are not consumed in proportionate volumes, thereby leaving a residual amount of either the fuel or the oxidizer in one of the propellant tanks near the end of the satellite's useful life. When only one propellant is left, no more propulsive force can be generated by combustion of the propellants.
Rocket thrusters are generally positioned at different locations on the satellite body with thrust nozzles pointed in different directions to position the satellite at a desired location in orbit (station keeping), or to control its orientation in a desired direction (attitude control). A Hughes HS-393 communications satellite that uses such thrusters for station keeping and attitude control is described in JCSAT-1 Propulsion Anomaly, Hughes Space and Communications Company, 1996. Simplified views of the attitude control and station keeping thrusters of the HS-393 and a similar model, HS-389, are shown in FIGS. 3a, 3b and 3c. FIG. 3a is a simplified perspective view of the satellite, comprising a substantially cylindrical body 38 that is spun about its cylindrical axis 40 for stabilization in orbit. A simplified top view of the satellite is shown in FIG. 3b. Four canted radial thrusters 42 are positioned on the side surface of the cylindrical body, with one pair pointing in one direction and another pair pointing in an opposite direction. The side surface of the cylindrical body is covered with a solar panel 46, which is cut out at designated locations 48 in FIG. 3a to allow the canted radial thruster nozzles to protrude from the side surface. Control of the satellite's longitude (east-west direction) in the geosynchronous orbit is achieved by firing a pair of canted radial thrusters on the same side. The rate of spinning is controlled by firing a canted radial thruster individually or two such thrusters in a diagonal pair.
The satellite also has a plurality of axial thrusters 44, which are fired to control the attitude (or orientation) of the satellite by adjusting the orientation of its spin axis 40. The axial thrusters and the canted radial thrusters each produce a relatively small thrust force of about 22 Newtons (N).
The bottom of the cylindrical satellite body includes a spun shelf 50, a perspective view of which is shown in FIG. 3c. Four additional canted radial thrusters 52 with a thrust force on the order of 22 N are provided near the edges of the spun shelf to provide spin stabilization. A cylindrical central thrust tube 54 extends from the center of the spun shelf and terminates in an open ended cone, with two axial thrusters 44 inside the tube and two apogee thrusters 56 outside. The apogee thrusters are used to lift the satellite up from a transfer orbit to the geosynchronous orbit and have a thrust force on the order of 490 N, which is much greater than the thrust force of the axial and radial thrusters.
Another type of geosynchronous satellite, an example of which is a Hughes HS-601 model illustrated in FIG. 4, is stabilized without spinning. FIG. 4 illustrates only a simplified perspective view of the spacecraft without showing its external antennas and solar panels, with which the present invention is not concerned. The satellite body 58 is substantially cubical with six planar surfaces. Two opposite surfaces 60 (one of which is not shown due to the perspective view) each have two east/west thrusters 62 to adjust the longitude of the satellite in geosynchronous orbit. Another pair of opposite surfaces 64 (one of which is not shown due to the perspective view) each have four north/south thrusters 66 positioned near the corners of each surface to adjust the latitude (or inclination) of the satellite. One of the remaining surfaces 68 has four axial thrusters 70 to control the attitude of the satellite by firing one or more of the thrusters. An apogee thruster 72 is positioned at the center of the surface 68 to propel the satellite from a transfer orbit to the geosynchronous orbit. After the satellite is transferred to the geosynchronous orbit, the apogee thruster is disabled.
Regardless of the type of satellite or thruster arrangement, thruster rockets with bipropellant propulsion must have both the oxidizer and the fuel available for combustion so that a thrust force can be produced. The position (latitude and longitude) and attitude of the satellite need to be adjusted in orbit for proper operation. When one of the two propellants is depleted, the rocket can no longer provide the thrust force for adjusting the position and/or attitude.